1. Field of the Invention
The present invention relates to a variable capacitance element included in a high-frequency circuit. In particular, the present invention relates to a variable capacitance element, for use as, for example, a variable capacitance switch or a variable capacitor. In this case, the variable capacitance switch performs switching operations on high-frequency signals by changing electrostatic capacitance.
2. Description of the Related Art
Generally, a variable capacitor is known. A variable capacitor displaces, for example, a movable electrode with respect to a fixed electrode by using electrostatic gravity so as to change the spacing between these electrodes. Thus, the electrostatic capacitance is variably selected.
This kind of conventional variable capacitor is substantially similar to an electrostatically-driven switch as disclosed in Japanese Unexamined Patent Application Publication No. 2000-188050. The variable capacitor includes a movable electrode on the right side of the substrate including a flexible supporting bar. The flexible supporting bar bends toward the front side of the substrate. The movable electrode is spaced from and faces toward the fixed electrode provided on the substrate. Driving electrodes are provided on the substrate side and the movable electrode side. Voltage is applied between the driving electrodes externally such that electrostatic gravity is produced.
When voltage is not applied between the driving electrodes, the supporting bar freely supports the movable electrode. Thus, a predetermined space (electrostatic capacitance) is set between the fixed electrode and the movable electrode. When voltage is applied between the driving electrodes, the supporting bar is bent and is deformed due to the electrostatic gravity such that the movable electrode is displaced toward the fixed electrode. Thus, the electrostatic capacitance between these electrodes increases.
FIG. 4A schematically shows an example of a shunt switch element, which is a variable capacitance element. A shunt switch element 130 includes a substrate 131 containing a dielectric. A coplanar line 132 is provided on the substrate 131. High-frequency signals are conducted through the coplanar line 132. Three lines 133g1, 133s and 133g2 are aligned at a desired interval on the substrate 131. The middle line 133s is a signal line. The lines 133g1 and 133g2 on both sides of the signal line 133s are ground lines.
On the coplanar line 132, both ends of an electrode bridge 134 are joined with the ground lines 133g1 and 133g2. The electrode bridge 134 crosses over the signal line 133s. FIG. 4B is a top view of FIG. 4A schematically showing the coplanar line 132 and the electrode bridge 134.
When direct-current voltage is applied between the signal line 133s and the electrode bridge 134 included in the shunt switch element 130, electrostatic gravity is produced between the signal line 133s and the electrode bridge 134. As a result, the electrode bridge 134 is pulled toward the signal line 133s due to the electrostatic gravity. Therefore, the electrostatic capacitance changes between the electrode bridge 134 and the signal line 133s of the coplanar line 132.
It is important to note that equivalent circuits of the coplanar line 132 and the electrode bridge 134 can be expressed as shown in FIG. 4C. In FIG. 4C, the reference letter C indicates an electrostatic capacitance between the signal line 133s and the electrode bridge 134. The reference letter L indicates an inductance component of the electrode bridge 134. The reference letter R indicates a resistance component of the electrode bridge 134.
When the space between the signal line 133s and the electrode bridge 134 is reduced, and when the electrostatic capacitance C between the signal line 133s and the electrode bridge 134 is increased, the self-oscillating frequency of the LC series circuit in FIG. 4C decreases. At the self-oscillating frequency of the LC series circuit, the impedance of the LC series circuits is minimized. Thus, when viewing the ground lines 133g1 and 133g2 from the signal line 133s through the electrode bridge 134, a short circuit occurs at high frequencies of the self-oscillating frequencies of the LC series circuit. As a result, the conducting of high frequency signals through the coplanar line 132 (signal line 133s) is turned OFF.
On the other hand, when the space between the signal line 133s and the electrode bridge 134 is increased and when the electrostatic capacitance C between the signal line 133s and the electrode bridge 134 is decreased, the self-oscillating frequency of the LC series circuit in FIG. 4C increases. As a result, when viewing from the signal line 133s through the electrode bridge 134, the ground lines 133g1 and 133g2 are open to high frequencies. Therefore, the conducting of high-frequency signals through the coplanar line 132 is turned ON.
As described above, the shunt switch element 130 controls ON and OFF of the conducting of high frequency signals through the coplanar line 132. In this case, the electrode bridge 134 is displaced and the electrostatic capacitance C is variable between the electrode bridge 134 and the signal line 133s. 
It is important to note that, in the above-described conventional technology, the movable electrode is held at a predetermined location by its own force (spring force) when voltage is not applied between the driving electrodes. However, when a variable capacitance element is used, an external force such as vibration and impact may be applied to the substrate. Thus, when external forces act thereon in a direction perpendicular to the substrate, the supporting bar is bent and is deformed by the external force. As a result, the movable electrode is displaced with respect to the fixed electrode.
Therefore, while the variable capacitor is operating, the electrostatic capacitance of the capacitor may be changed due to vibration and/or impact even without voltage being applied to the driving electrode. Therefore, the vibration resistance and reliability are reduced. This is a disadvantage.
Furthermore, in the conventional variable capacitance element, when two driving electrodes are too close to each other and contact each other, they remain fixed together even after the voltage is terminated. In this case, returning the variable capacitance element to the normal operating state is difficult using only the stability of the supporting bar. This is another disadvantage.
In the construction of the shunt switch element 130, the electrode bridge 134 functions as both a driving electrode and an electrode for electrostatic capacitance. In this case, the driving electrode is paired with a fixed driving electrode to produce electrostatic gravity. The electrode for electrostatic capacitance is paired with the signal line 133s to determine the self-oscillating frequency of the LC series circuit shown in FIG. 4C.
However, when high frequency signals are conducted through the coplanar line 132, such as extremely high frequency (EHF) signals, the amount of electrode surface area of the electrode bridge 134 must be reduced. Thus, the conducting of high frequency signals through the coplanar line 132 can be turned ON and OFF precisely by using changes in the self-oscillating frequency of the LC series circuit using the change in electrostatic capacitance as described above. On the other hand, when the electrode bridge 134 is small, large direct-current voltages must be applied between the electrode bridge 134 and the fixed driving electrode in order to produce electrostatic gravity for displacing the electrode bridge 134. However, the electrode bridge 134 is preferably displaced by using low direct-current voltage. Therefore, from the viewpoint of the displacement driving, the size of the electrode bridge 134 is preferably increased.
As described above, the size of the electrode bridge 134 for controlling ON and OFF of the conducting of high frequency signals is different from the size of the electrode bridge 134 that is suitable for displacement of the electrode bridge 134 itself. Thus, designing the electrode bridge 134 is difficult.
To overcome the problems described above, preferred embodiments of the present invention provide a variable capacitance element which prevents the displacement of a movable electrode due to an external force and/or the fixing of a driving electrode, which stabilizes the operation of elements and which greatly improves vibration resistance and reliability.
Preferred embodiments of the present invention also provide a variable capacitance element which greatly improves the degree of freedom of electrode design.
According to a preferred embodiment of the present invention, a variable capacitance element includes a substrate, a high frequency signal conducting portion on the substrate, a movable body located above and spaced from the substrate and facing toward at least a portion of the high frequency signal conducting portion, a movable electrode provided on the movable body and facing toward the high frequency signal conducting portion, a fixed side electrode provided on the substrate for movable body displacement, in an area facing toward the movable body and spaced from the high frequency signal conducting portion, the movable body includes an insulating semiconductor or an insulator against high frequency signals, and a movable side electrode for movable body displacement provided on a substrate facing surface of the movable body, facing toward the fixed side electrode for movable body displacement on the substrate, and spaced from the movable electrode. The movable side electrode for movable-body displacement and the fixed side electrode for movable body displacement are included in a capacitance variable unit, which displaces the movable body toward the substrate by using electrostatic gravity produced by applying direct-current voltage between the movable side electrode for movable body displacement and the fixed side electrode for movable body displacement so as to vary electrostatic capacitance between the movable electrode in the movable body and the high frequency signal conducting portion on the substrate.
The variable capacitance element preferably further includes an upper member provided above and spaced from the movable body. The movable side electrode for movable body displacement is preferably provided on an upper member surface facing toward the movable body instead of on the substrate facing surface of the movable body. The fixed side electrode for movable body displacement is preferably provided on the upper member and faces toward the movable side electrode for movable body displacement instead of on the substrate. The movable side electrode for movable body displacement and the fixed side electrode for movable body displacement are preferably included in a capacitance variable unit, which displaces the movable body toward the upper member by using electrostatic gravity produced by applying direct-current voltage between the movable side electrode for movable body displacement and the fixed side electrode for movable body displacement so as to vary electrostatic capacitance between the movable electrode in the movable body and the high frequency signal conducting portion on the substrate.
The high frequency signal conducting portion is preferably one of a coplanar line and a microstrip line. The variable capacitance element is preferably a shunt switch element for controlling ON and OFF of signal conduction of the coplanar line or the microstrip line, which is the high frequency signal conducting portion, by using a change in electrostatic capacitance between the movable electrode and the high frequency signal conducting portion.
According to another preferred embodiment of the present invention, a variable capacitance element includes a substrate, a high frequency signal conducting portion provided on the substrate for conducting signals from the outside, a movable electrode movably provided on the substrate for switching conduction states for signals conducting to the high frequency signal conducting portion by moving toward or away from the high frequency signal conducting portion, a first driving electrode provided on the substrate and facing toward the movable electrode for displacing the movable electrode to a first switching position near the high frequency signal conducting portion by the conduction of high frequency signals, and a second driving electrode provided on the opposite side of the first driving electrode through the movable electrode for displacing the movable electrode to a second switching position away from the high frequency signal conducting portion by the conduction of high frequency signals.
With this construction, the movable electrode is provided between the first and second driving electrodes. Therefore, when voltage is applied to one of the first and second driving electrodes, the movable electrode is forcibly displaced to the first switching position or to the second switching position using the electrostatic gravity. Thus, the movable electrode is driven in both directions with respect to the non-conducting position. As a result, even when external forces, such as vibration and impact, are applied to the substrate, the conducting and shutting states of the high frequency signal conducting portion for high frequency signals are switched in accordance with the position of the movable electrode.
Preferably, the variable capacitance element further includes a third driving electrode provided in a portion of the movable body, facing toward the second driving electrode. The high frequency signal conducting portion is preferably located near the first driving electrode on the substrate. The movable body is located between the first driving electrode and the second driving electrode, for displacement between the first and second driving electrodes. The movable electrode is preferably located in a portion of the movable body, facing toward the high frequency signal conducting portion and the first driving electrode.
Thus, the movable electrode is movably supported by the movable body. The first driving electrode and the movable electrode are preferably located on one side with respect to the movable body. The second and third driving electrodes are preferably located on the other side. When voltage is applied between the electrodes on one side or between the electrodes on the other side, the movable electrodes are moved toward or away from the high frequency signal conducting portion.
Preferably, the variable capacitance element further includes a stopper for securely holding the movable electrode at the first switching position when the movable electrode is displaced by the first driving electrode.
Thus, when the movable electrode reaches the first switching position via electrostatic energy, the stopper provided in the substrate, the high frequency signal conducting portion and/or the first driving electrode abut with the movable electrode. Alternatively, the stopper provided in the movable electrode can abut with the substrate, the high frequency signal conducting portion and/or the first driving electrode. Therefore, the movable electrode is securely held.
The stopper may be provided in the movable electrode. The stopper may abut with the high frequency signal conducting portion when the movable electrode is displaced to the first switching position.
Thus, when the movable electrode reaches the first switching position, the stopper abuts with the high frequency signal conducting portion. Therefore, the movable electrode is securely held at the first switching position. The movable electrode and the high frequency signal conducting portion are preferably insulated by the stopper.
The stopper preferably includes a dielectric material. Thus, the stopper abuts with the high frequency signal conducting portion at the first switching position. At the second switching position, the stopper is moved away from the high frequency signal conducting portion. As a result, a capacitance component between the conductors included in the high frequency signal conducting portion are increased more at the first switching position in accordance with the dielectric constant of the dielectric material than at the second switching position. Therefore, high frequency signals to be conducted to the high frequency signal conducting portion are securely blocked.
The high frequency signal conducting portion preferably has a larger thickness than that of the first driving electrode in the front surface side of the substrate. Thus, at the first switching position, the stopper in the movable electrode side does not abut against the driving electrode. As a result, the stopper securely abuts against the high frequency signal conducting portion.
The stopper may be provided in the substrate or in the first driving electrode. The movable electrode may abut with the stopper when the movable electrode is displaced to the first switching position.
Thus, when the movable electrode reaches the first switching position, the movable electrode abuts with the stopper. Therefore, the movable electrode is securely held at the first switching position. As a result, the stopper insulates the movable electrode from the high frequency conducting portion.
Preferably, a stopper is provided on the movable body. The stopper securely holds the movable electrode in the first switching position when the movable electrode is displaced by the first driving electrode.
Thus, when the movable electrode reaches the first switching position, the stopper provided in the movable body abuts with the substrate and/or the first driving electrode. Therefore, the movable electrode is securely held at the first switching position. As a result, the stopper insulates the movable electrode from the high frequency conducting portion.
Preferably, the variable capacitance element preferably further includes another stopper for holding the movable electrode steady at the second switching position when the movable electrode is displaced by the second driving electrode.
Thus, when the movable electrode reaches the second switching position, another stopper abuts with the periphery. Therefore, the movable electrode is securely held in the second switching position. As a result, the movable electrode is prevented from displacing due to vibration or impact.
According to another preferred of the present invention, a variable capacitance element includes a substrate, a high frequency signal conducting portion provided on the substrate, a movable electrode movably provided on the substrate for changing the electrostatic capacitance between the movable electrode and the high frequency signal conducting portion by displacement toward or away from the high frequency signal conducting portion, a first driving electrode provided on the substrate and facing the movable electrode for displacing the movable electrode toward the high frequency signal conducting portion, and a second driving electrode provided on the opposite side of the first driving electrode through the movable electrode for displacing the movable electrode away from the high frequency signal conducting portion.
Thus, the movable electrode is provided between the first and second driving electrodes. These driving electrodes forcibly displace the movable electrode in both directions with respect to the non-conducting position by using electrostatic gravity. Therefore, even when external forces, such as vibration and impact, are applied to the substrate, voltage to be applied to the first and second driving electrodes is separately controlled. Thus, electrostatic capacitance between the high frequency signal conducting portion and the movable electrode can be changed with stability.
The variable capacitance element preferably further includes a voltage control unit for separately controlling magnitudes of voltage applied to the first and second driving electrode, respectively. Thus, the voltage control unit controls the relationship and voltage ratio of the voltages to be applied to the first and second driving electrodes, respectively. Therefore, the movable electrode is precisely driven in a wide range. As a result, the electrostatic capacitance between the high frequency signal conducting portion and the movable electrode can be changed continuously.
According to various preferred embodiments of the present invention, a movable electrode and a movable side electrode for movable body displacement are separately provided. The movable electrode is paired with a high frequency signal conducting portion. Electrostatic capacitance is produced between the movable electrode and the high frequency signal conducting portion. The movable side electrode for movable body displacement displaces a movable body including the movable electrode using electrostatic gravity.
Conventionally, an electrode (electrode bridge) is provided which functions as both an electrode (that is, a movable electrode) for causing electrostatic capacitance with the high frequency signal conducting portion and as an electrode (that is, a movable side electrode for movable body displacement) for displacing the electrode. Designing the electrode bridge has many constraints for achieving both of the functions. Therefore, the electrode design is very limited.
On the other hand, according to preferred embodiments of the present invention, the functions are implemented separately by the movable electrode and the movable side electrode for movable body displacement. Therefore, the movable electrode and the movable side electrode for movable body displacement can be designed independently. As a result, the flexibility in electrode design is greatly improved.
Conventionally, the electrode bridge itself is bent and is deformed to change electrostatic capacitance between a high frequency signal conducting portion and the electrode bridge. Therefore, metal in the electrode bridge is easily fatigued. On the other hand, according to preferred embodiments of the present invention, the movable electrode and the movable side electrode for movable body displacement are provided in the movable body. Furthermore, the movable body may include a flexible and insulating material other than a metal material. Therefore, the deterioration due to displacement of the movable body and/or the metal fatigue of the movable electrode and the movable side electrode for movable body displacement does not occur. As a result, the durability of the movable capacitance element is greatly improved.
Furthermore, the movable side electrode for movable body displacement is provided on a substrate facing surface of the movable body. Additionally, a fixed side electrode for movable body displacement is provided on the substrate. Therefore, an upper member is not required for mounting the fixed side electrode for movable body displacement. That is, the upper member can be removed. As a result, the construction and steps of manufacturing the variable capacitance element are greatly simplified.
In addition to the removal of the upper member, the movable body can be displaced toward the substrate using electrostatic gravity produced between the movable side electrode for movable body displacement and the fixed side electrode for movable body displacement. Therefore, the movable body is not moved upward away from the substrate by the electrostatic gravity. As a result, the height of the variable capacitance element is greatly reduced.
The high frequency signal conducting portion is preferably a coplanar line or a microstrip line. The variable capacitance element is preferably a shunt switch element. In this case, in order to precisely control ON and OFF of the signal conduction of the high frequency signal conducting portion, the movable electrode preferably has a small electrode surface in accordance with the high frequency of high frequency signals flowing through the high frequency signal conducting portion. On the other hand, in order to displace the movable body at low voltages, the electrode surface is preferably large where the movable side electrode for movable body displacement and the fixed side electrode for movable body displacement face toward each other.
According to various preferred embodiments of the present invention, the movable electrode and the movable side electrode for movable body displacement can be designed independently. Therefore, the movable electrode can be designed to have a size that is suitable for controlling ON and OFF of the signal conduction of the high frequency signal conducting portion. Thus, a shunt switch element is easily provided, which has greatly improved performance and which precisely controls ON and OFF of the conducting of high frequency signals with a reduced voltage supply.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.